Power transmission line systems typically include multiple buses of high voltage. It is a common practice to transfer portions of the transmission line system to which power users are connected from one high voltage bus to another, while maintaining the actual current load through the circuit breaker to the transmission line. During such a transfer between two buses, the high voltage source, referred to as the source feed, is thus changed without interrupting power to the load. This is accomplished by first connecting the original bus supplying high voltage and a second bus in parallel and then opening the original bus. The result is a continuous feed of high voltage load current to the customer.
Since this is a normal part of the operation of a transmission line system, the transfer of the high voltage potential feed from one bus to another should not in and of itself cause an alarm output from any of the protective relays for the system which would in turn result in the opening of a circuit breaker protecting that portion of the system. Such an alarm output from a protective relay is typically referred to as a trip signal.
In fact, however, such a trip signal may result from such a bus transfer, during periods of high current flow, unless the transfer is accompanied by corrective action taken in conjunction with the operation of the transfer switch.
A trip signal may occur because the transfer switch operates on "a break before make" basis. Hence, there will actually be a momentary loss of polarizing potential to the protective relay before the transfer to the new bus is completed. This momentary loss of potential, accompanied by high current flow through the protective relay and the associated circuit breaker, will be recognized by the protective relay as a fault condition and it will subsequently produce a trip signal. However, that trip signal, which results in the opening of the breaker, is undesirable because it is not in fact indicative of a true fault condition and results in an interruption of power to the customers serviced by the transmission line.
The general problem of detecting such a loss-of-potential condition and thereafter blocking the opening of the breaker by means of a trip signal output from the protective relay has been addressed by what is known as a loss-of-potential (LOP) circuit, which recognizes a loss-of-potential condition which is not indicative of a true fault. The loss-of-potential circuit output is used in effect to suppress the trip output from the distance elements of the relay, so that the circuit breaker will not open and power is maintained to the end user customers. However, in some cases, a "race" condition results between the LOP output and selected distance (or other) protective elements, the result of which an LOP condition is interpreted as a true fault condition and the circuit breaker is opened. One solution to this problem is to provide a five second time delay for the LOP "set" signal, to prevent the LOP logic circuit from blocking relay tripping for true fault conditions. Such a solution does result, however, in the LOP logic not being effective in blocking the operation of the relay during polarizing voltage transfer operations.
Another alternative solution is to slow down the distance relay logic. However, the rapid detection of system faults is critical, and hence any slow down in the detection of those faults is not acceptable. Hence, there remains a need for a clear and reliable differentiation between a loss-of-potential condition due to the operation of a potential transfer switch and a true fault condition for which a circuit breaker should be operated.